Defects in DNA repair have been hypothesized to drive the accumulation of DNA damage that leads to aging and age-associated diseases, such as cancer. The observations that DNA damage accrues with age and that aspects of DNA repair pathways are altered in models of normal aging have been made in several organisms. These studies, however, are correlative and the chronology of events and the mechanism of aging remain unknown. Accordingly, I propose to characterize the role of DNA repair defects in aging and lifespan determination using three distinct approaches. Specific aim 1 explores DNA damage and the role of DNA repair in the context of normal aging by determining the differences in mutation rate, mutation spectrum and oxidative damage between young and old cells. The goal of specific aim 2 is to investigate how cellular response to various types of extrinsic damage changes or becomes attenuated in an age-dependent manner. This will be accomplished by subjecting young and old cells to DNA damaging agents and identifying differences in sensitivity and resistance. Specific aim 3 utilizes the characterized yeast genetic interactions network and tetracycline repressible gene expression system to identify DNA repair pathways that become deficient with age. Saccharomyces cerevisiae is the ideal organism for accomplishing these aims because it is a well characterized and tractable system whose replicative lifespan is a model for aging. Greater understanding of the role of DNA repair pathways in the aging process will shed light onto the fundamental mechanisms of both normal and pathological aging. With the advent of modern medicine, the average lifespan of the human population continues to increase. These advancements are not without consequence, as we are now faced with new challenges of age-associated morbidities such as cancer and neurodegeneration. Connections between genetic instability, DNA repair, aging, and age-dependent diseases has been established, but refinement of these relationships are required to reveal potential targets for disease prevention, detection, and clinical intervention.